This invention relates to temperature measurement in general, and more particularly to an improved bridge circuit for providing a temperature proportional output.
With the increasing use of digital voltmeters in measurement and automation, along with the acquisition of measurement data controlled by process computers, transducers whose output voltages are proportional to the measurement quantity converted into an electrical signal are required in order to avoid the burdensome conversions.
For use in temperature measurements, the most popular temperature transducers aside from thermocouples, are resistance thermometers having a characteristic R(t) = R.sub.0 (1 + .alpha.t+ .beta. t.sup. 2 + .gamma. t.sup. 3). These are commonly employed in a bridge circuit. If the bridge operates using a deflection system, the voltage generated in the bridge diagonal is not strictly proportional to the measured temperature. This is a well known phenomenon [See ATM J 910, Oct. 14, 1968, pp. 217 to 220.] However, it can be corrected if indicating meters are used by using a non-linear scale. Since this is not possible when using digital voltmeters, circuit must be provided to linearize the readings. It is well known that the quadratically temperature dependent term in the diagonal bridge can be eliminated by suitable design of the simple Wheatstone bridge, using nickel resistance thermometers. It is also known that this is not a simple possibility if instead the more commonly used platinum resistance thermometers are used. As is well known, these are more important because of their reproducibility and larger permissible temperature range.
Thus, complex circuits are required to linearize the outputs of bridges using platinum resistance thermometers. In the above cited literature reference, such a circuit using two platinum resistance thermometers and multiple bridges is shown but without any design procedures and attainable results given. A multiple bridge system is also known from German Auslegeschrift 1,182,873, which describes the temperature measuring circuit with linear indications. Another circuit arrangement for linearizing the characteristics of thermo resistors by means of active circuit elements with negative feedback elements is shown in German Auslegeschrift 1,138,251. In this arrangement, the non-linearity of the resistance change is reduced in a negative-feedback circuit using transistors.
Another prior art resistance thermometer described in German Offenlegungsschrift 1,573,167 teaches linearizing using suitable interconnection of resistances with opposite sign for the resistance term which is a square-law function of temperature. This method is not suited for a bridge circuit, since even a resistance depending linearly on the temperature leads to a bridge voltage which is not temperature proportional. The combination of different materials required for this proposed linearization narrows the combination to the temperature range permissible with nickel, if standard resistors are to be used. In the absence of further parameters, the combination is further fundamentally limited to the elimination of the terms which depend on the temperature quadratically. Another prior art temperature measuring amplifier which is described in the magazine "Electronic Journal" of June, 1970, No. 6, pp. 48 to 51, is subject to the same limitation. In the arrangement disclosed therein, a voltage proportional to the resistance increase is squared in a multiplier and provided with a suitable scale factor, and then added to the voltage, since the cubic term depending on the temperature can fundamentally not be eliminated in the output voltage if the square term is to vanish.
Thus, it can be seen that there is a need for a temperature measuring arrangement of this nature in which a simply designed bridge can be used for the temperature measurement with the diagonal voltage of the bridge having both square and cubic temperature dependent terms which vanish, without requiring the use of expensive circuit units such as multipliers.